Reciprocal Interaction between TRAF6 and Notch Signaling Regulates Adult Myofiber Regeneration upon Injury

Abstract

Skeletal muscle is a postmitotic tissue that repairs and regenerates through activation of a population of stem-cell-like satellite cells. However, signaling mechanisms governing adult skeletal muscle regeneration remain less understood. In the present study, we have investigated the role of tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), an adaptor protein involved in receptor-mediated activation of multiple signaling pathways in regeneration of adult myofibers. Skeletal musclespecific depletion of TRAF6 in mice (TRAF6 mko ) improved regeneration of myofibers upon injury with a concomitant increase in the number of satellite cells and activation of the Notch signaling pathway. Ex vivo cultures of TRAF6 mko myofiber explants demonstrated an increase in the proliferative capacity of myofiber-associated satellite cells accompanied by an upregulation of Notch ligands. Deletion of TRAF6 also inhibited the activity of transcription factor NF-B and the expression of inflammatory cytokines and augmented the M2c macrophage phenotype in injured muscle tissues. Collectively, our study demonstrates that specific inhibition of TRAF6 improves satellite cell activation and skeletal muscle regeneration through upregulation of Notch signaling and reducing the inflammatory repertoire.

Signaling mechanisms in regenerative myogenesis.

“…Ablation of TRAF6 did not have any major effect on the phosphorylation of Akt kinase and p38MAPK (84). While these data are in contrast to previous reports of TRAF6-dependent activation of Akt (36) and p38MAPK (101), they highlight TRAF6-mediated differential activation of downstream signaling pathways (102).…”

“…Studies from our lab has also shown that blocking TRAF6 in differentiated myofibers inhibits the activation of NF-B and increases levels of promyogenic M2c macrophages in regenerating skeletal muscle potentially through a cross-talk with Notch pathway (84). This suggests that the inhibition of TRAF6 limits the levels of inflammatory cytokines which hastens the appearance of M2c macrophages resulting in increased proliferation of myogenic cells and rapid restoration of myofibers architecture.…”

“…We have also recently reported that inhibition of TRAF6 improves myofiber regeneration upon cardiotoxin-mediated injury (84). We next sought to investigate whether the inhibition of TRAF6 improves muscle regeneration in mdx mice.…”

“…Accumulating evidence suggests that TRAF6 is a crucial regulator of skeletal muscle mass in catabolic states (41,43,84). Abundance and activation of TRAF6 are increased in skeletal muscle of mice in many atrophying conditions.…”

“…Notably, inhibition of TRAF6 using genetic approaches attenuates muscle-wasting in response to denervation, cancer cachexia, and starvation (41,43). Furthermore, specific inhibition of TRAF6 also improves myofiber regeneration upon cardiotoxin-mediated injury (84). One of the mechanisms by which TRAF6 mediates muscle-wasting is through stimulation of autophagy (41,43).…”